In recent years, renewable energy of sunlight, wind power, or the like has attracted attention as safe and clean energy, and an increase in introduction quantity thereof is expected in the future. However, such renewable energy has a low operation rate and a large output fluctuation in a short time, and has a problem in terms of stable supply. Further, if a large quantity of such renewable energy is introduced, a problem arises in that the introduced energy is not completely consumed and remains as surplus power. In this regard, there is a demand for development of techniques of storing electric power such that electric power can be stably supplied and only a necessary amount of electric power can be supplied whenever it is needed even if the introduction of such renewable energy increases. For storing electric power, in addition to a method of storing electric power in the form of electricity, a method of converting electric power into chemical energy and storing the chemical energy has been investigated. In particular, the method of storing electric power in the form of chemical energy has advantages in that the chemical power can be stored in the unit of several days to weeks or a longer span and the chemical power can be transported and used at a different place as necessary. Recently, a method of storing the electric power in the form of hydrogen attracts attention; however, methane or methanol, which is excellent in volumetric energy density as compared with hydrogen, is also a major candidate. In particular, regarding methane or the like, there are a plenty of devices capable of directly using methane or the like as a fuel and the infrastructure therefor is also established.
For example, reaction (1) in which methane is synthesized from hydrogen (H2) obtainable by electrolyzing water by renewable energy and carbon dioxide (CO2) and reaction (2) in which methanol is synthesized from hydrogen (H2) and carbon monoxide (CO) are mentioned.CO2+4H2→CH4+2H2O  (1)CO+2H2→CH3OH  (2)
This reaction (1) called Sabatier reaction is a reaction for reductive regeneration of CO2 that is one of causes of global warming, and since the reaction is performed at a relatively low temperature of about 400° C., a great deal of research has hitherto been conducted on this reaction.
According to this reaction, the conversion of CO2 and the yield of methane can be increased as the reaction approaches equilibrium and a lower temperature region; however, the reaction rate is decreased, and thus, it is more difficult to put this reaction into practical use. For this reason, when the reaction is carried out at low temperature, a noble metal-based catalyst is necessary. However, the noble metal-based catalyst is expensive. Meanwhile, when the temperature is increased to about 400° C., the reaction rate is increased and a Ni-based catalyst can be used. However, in this case, a by-product such as CO is also generated, and as a result, energy is consumed for separation of the by-product, or the like. In this regard, there is a demand for development of a non-noble metal-based catalyst having high activity at a lower temperature region and high methane yield.
As a catalyst having high activity at low temperature of the related art, for example, a Ni-based catalyst having ZrO2 or CeO2 as a base material has been known. It is known that a Ni catalyst supported on a CeO2 base material has high reaction activity at low temperature. It is considered that these base materials have oxygen defects and help CO or CO2 to be easily dissociated at lower temperature and cause the reaction with hydrogen at low temperature to be performed effectively.
However, these catalysts have metal particles supported on an oxide base material that is powder and thus are difficult to handle without any change, and it is necessary to granulate these catalysts in a suitable size using a binder or the like. Further, there is also a problem in durability such as weak binding force between the metal particles and the oxide base material. In particular, since methanation reaction is exothermic reaction, heat resistance to withstand a local temperature increase is also required.